It is well-known that oscillations (vibrations) of objects are measured using sensors such as accelerometers, linear velocity displacement transducers and displacement gauges. These methods measure motions locally at a few discrete locations through contact with the surface. Due to their mass, such sensors can affect the response of the object being measured.
In addition to the effects of mass on the response of an object, such methods typically measure object motions and deformations only along a specific direction and at discrete points. To obtain measurements of motion in all directions at a point, either a combination of several sensors located at the same point or a combination of several experiments with sensors oriented in distinct directions at the same point is required to obtain all of the motion components at a given point. Even if multiple sensors are used, a measurement of the deformations of the object surface caused by the oscillations cannot be determined using motion data at a single position since the gradients of the deformation may also be required. Due to the size and weight of these sensors, it is not possible to place additional sensors sufficiently near the same point to acquire accurate measurements of the surface deformations.
These deficiencies have led to the development of optical non-contacting measurements. Optical measurement methods do not contact the surface and as such do not affect the response of the object. In one such method, laser vibrometers are capable of acquiring motion measurements for vibrating objects without contacting the surface. In its standard form, a laser vibrometer acquires measurements at one point.
A scanning laser vibrometer can operate in a manner that scans across the object, acquiring motion measurements at several positions on the object surface. A disadvantage of the method is that the scan time increases according to the density of the measuring points. A further disadvantage of any scanning laser vibrometer is the missing reference to an object point for the measurement of relative object motions between points on the object surface. The primary quantity measured by laser vibrometers is the relative phase change and/or the rate of change due to the optical path length variation induced by object surface motions. The sensitivity direction is given by the combination of illumination and observation angle. That is, measurements are made along a line of sight without direct reference to a fixed object point. Therefore, a measurement of the relative motion of two object points is impossible and strain measurements cannot be obtained in the general case. A further disadvantage are the high costs due to the use of expensive optical components, coherent light sources, vibration isolation components, and the requirements to have a highly reflective object surface during the measurement process.
Additional non-contacting measurement methods include Speckle interferometry, such as speckle holography or speckle shearography, used to obtain full-field (total visible surface) motion measurements during object vibrations and/or oscillations. These methods can provide a direct reference to the object surface and thus, determination of object strains is possible. A major disadvantage of these procedures is that the coherent illumination and measurement process can only be used to measure small object motions due to the high sensitivity of interferometric methods. Additional disadvantages include the deleterious effects of: (a) small environment disturbances; and (b) rigid body motion of the object relative to the recording medium. A further disadvantage is the high cost due to the use of expensive optical components and coherent light sources.
Yet another non-contacting measurement method includes digital speckle photography or digital image correlation originally developed to measure the 2D deformations of an object subjected to a change in loading (i.e. static loading change). The method stores images of a randomly varying intensity pattern in the two loading states and uses software to compare sub-regions in each pattern to extract full-field measurements of surface displacement. When combined with multiple cameras and appropriate mathematical formulation, the method is capable of determining full field 3D surface motions. The random pattern provides a locally unique set of markers to allow for determination of correspondences between many small sub-sets within the image so that it is possible to measure a full-field of local surface deformations. Known as a speckle pattern, the randomly varying intensity field may be naturally occurring or artificially applied.